It is known to incorporate a bar code in a paper in the fashion of a watermark, in particular in a security paper, for manufacturing security documents or documents of value. For example, the authenticity of bank notes can be tested with reference to the bar code, if all bank notes of a certain value and a certain date of issue bear a certain watermark in the same fashion. By comparing with the serial number and the denomination printed on the bank note then the authenticity of the bank note can be checked with reference to the bar code. Depending on the complexity of the information encoded in the watermark bar code, the bar code can become comparatively long, which is undesirable with small dimensioned documents of value, such as for example bank notes.
The problem, however, is not only the length of the bar code, but in particular its checkability for the purpose of determining the authenticity. Because the broader the bars incorporated as a watermark in the paper, the more irregular is their appearance on transmissive viewing. The reason for this is that a watermark with a regular dark surface can be realized only with difficulties. For manufacturing the watermark the papermaking screen is embossed, so that the deposit of paper fiber during the sheet formation is influenced. If the papermaking screen is deep-embossed, more paper pulp deposits in this area, while a high-embossing impedes the deposit of paper pulp. With an embossed surface above all the edges of the surface are reproduced well. The surface itself towards its inner area becomes either lighter or darker.